Multi-plate clutches are also well known assemblies, in which plates that are directly or indirectly connected to a drive shaft rotate with respect to plates fixed on the housing. These plate packs can be moved in the axial direction against a spring force or any other preload, for example by means of hydraulic force, such that the distance between the rotating and the stationary plates is variable. In full contact a power transfer (rotational speed×torque) takes place and in case of a complete detachment, the drive and the output are separated from each other in an end position which offers a sufficient distance. Intermediate stages can be controlled in usual installations by means of a corresponding control of the pressure medium.
As soon as a drive starts and makes a drive shaft rotate, the drive shaft of the clutch unit will be set in rotation. The multi-plate clutch will be engaged by a corresponding pressurization by means of a pressure medium, for example pressure oil, and thus a corresponding transmission of the rotation will be enabled.
It is known that only by varying the pressure on the clutch piston, an essentially continuous speed adjustment of the output side with respect to the drive speed can take place without even influencing the drive speed. Auxiliary units for providing lubricating oil, cooling oil medium and the like are of course provided.
It is known that in the low speed ranges of combustion engines this speed adjustment has its limit. Speeds beneath the motor idle speeds are usually not possible. The use of an adjustable multi-plate clutch makes it however possible to operate this one with a low engagement pressure, wherein one calls this a “slipping clutch”. In this operating state, there is no synchronous run between the drive and the output side. This is for example the case, if the engine has to be operated for other reasons within a correspondingly high speed range. Such reasons can be the provision of power for safety devices for fire-fighting and the like.
The slipping of the clutches leads to a power loss.
The cooling capacity is of a correspondingly high importance. The power loss arising in the slipping operation is dissipated from the clutch via the internally supplied quantity of oil. This energy input has to be dissipated from the system via a cooler. Usually this is realized by means of a water-oil heat exchanger. Apart from the quantity of supplied cooling medium and the entry temperature of the cooling medium, the quantity of oil which is made available on the side of the clutch is correspondingly decisive. High quantities of oil are required for dissipating high power losses. As it is however in the practical operation of the units impossible to predict when the maximum power loss will take place, a correspondingly high amount of cooling oil has to be pending. If the clutch is open, this high quantity of oil however causes the generation of high drag torques which can lead to correspondingly high speeds on the output side. These speeds are however usually not desired if the clutch is open. The aim would be the continuous speed adjustment until zero.
A possible remedy is a holding brake, by means of which the output side is definitely fixed. But an uncontrollable speed range will remain.